J . Am. Chem. Sot. 1984, 106, 3423-3429
3423
Catalysis of the Reduction of Molecular Oxygen to Water at Prussian Blue Modified Electrodes Kingo Itaya,* Nobuyoshi Shoji, and Isamu Uchida Contributionfrom the Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Sendai 980, Japan. Received September 28, 1983
Abstract: The reduced form of Prussian blues, Prussian white, does have a catalytic activity for the reduction of molecular oxygen and of hydrogen peroxide in aqueous acidic electrolytes. Rotating ring-disk voltammetric measurements show clearly that the reduction product of molecular oxygen is not hydrogen peroxide but water produced by the four-electron-transfer reaction. The oxidized form of Prussian blues also acts as a catalyst for the oxidation of hydrogen peroxide. Two kinds of electron transfer channels in the Prussian blue crystal due to the redox reactions of the high-spin ions, Fe3+/2+,and the low-spin iron ions, Fe"*/", work as a catalyst for the reduction and oxidation of hydrogen peroxide, respectively. The results seem to demonstrate the importance of zeolitic natures and of metal centers in the crystal of Prussian blues.
The search for a relatively inexpensive catalyst/electrode combination for the electrochemical reduction of molecular oxygen (0,)has received great interest because of the importance of fuel cells and air batteries.],, The catalytic properties of metal phthalocyanines and porphyrins for this purpose have been most extensively e ~ a m i n e d . ~ "Recently Collman et al. demonstrated the electrode catalytic activity of dicobalt "face-to-face" porphyrim6 The dicobalt cofacial porphyrin linked by four-atom bridges (the four-atom dimer) produced a catalyzed reduction almost exclusively to water. It has been pointed out that the 0, reduction process was extremely sensitive to the geometry of the dimers. The five- and six-atom dimers were less potent than the four-atom dimer.6b The above results suggest clearly that each of two metal centers in a suitable geometry can transfer two electrons to an oxygen molecule, carrying out the four-electron process. The best situation for the reduction of 0, seems to be that an oxygen molecule should be surrounded by either two- or four-electron sources. Such situations have been frequently discussed for cytochromes in living systems.' Recently both the present authors and Neff have described the fundamental nature of Prussian blue (PB) and its relatives by means of e l e c t r o ~ h e m i s t r y . ~The ~ ~ crystal structure of PB is a face-centered cubic lattice with a cell constant of 10.2 A, indicating a very roomy crystal structure.I0 PB has been known as a zeolite
(1) Yeager, E. J. Electrochem. Soc. 1981,128, 160c and references cited therein. (2) Jahnke, H.; Schonborn, M.; Zimmerman, G. Top. Curr. Chem. 1976, 61, 133. (3) (a) Fujihira, M.; Sunakawa, K.; Osa, T.; Kuwana, T., J . Electroanal. Chem., Inferfacial Elecfrochem. 1978, 88, 299. (b) Kobayashi, N.; Fujihira, M.; Sunakawa, K.; Osa, T. Ibid. 1979,101,269. (c) Kobayashi, N.; Matsue, T.; Fujihira, M.; Osa, T. Ibid. 1979, 103, 427. (4) (a) Bettelheim, A.; Chan, R. J. H.; Kuwana, T. J . Elecfroanal. Chem., Interfacial Electrochem. 1980, 110,93. (b) Forshey, P. A.; Kuwana, T. Inorg. Chem. 1983, 22, 699. (5) (a) Durand, R.; Anson, F. C. J. Electroanal. Chem., Inferfacial Electrochem. 1982,134,273. (b) Shigehara, K.; Anson, F. C. J. Phys. Chem. 1982,86, 2776.
( 6 ) (a) Collman, J. P.; Marrocco, M.; Denisevich, P.; Koval, C.; Anson, F. C. J. Electroanal. Chem., Inferfacial Elecfrochem. 1979, 101, 117. (b) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C. J . Am. Chem. Soc. 1980,102,6027. (c) Durand, R.; Bencosme, C. S.; Collman, J. P.; Anson, F. C . Ibid. 1983, 105, 2710. (7) Boyer, P. D.; Lardy, H.; Myrbick, K. 'The Enzymes"; Academic Press: New York, 1963. (8) (a) Itaya, K.; Shibayama, K.; Akahoshi, H.; Toshima, S. J. Appl. Phys. 1982,53,804. (b) Itaya, K.; Akahoshi, H.; Toshima, S . J . Electrochem. SOC. 1982, 129, 1498. (c) Itaya, K.; Ataka, T.; Toshima, S. J . Am. Chem. SOC. 1982,104,4767. (d) Itaya, K.; Ataka, T.; Toshima, S.; Shinohara, T. J . Phys. Chem. 1982,86, 2415. (e) Itaya, K.; Uchida, I.; Toshima, S. J . Phys. Chem. 1983, 87, 105. (9) (a) Neff, V. D. J . Elecfrochem. SOC.1978, 125, 886. (b) Ellis, D.; Eckhoff, M.; Neff, V. D. J . Phys. Chem. 1981, 85, 1225. (c) Rajan, K. P.; Neff, V. D. Ibid. 1982, 86, 4361.
0002-7863/84/1506-3423$01.50/0
with channel diameters of about 3.2 A. Our previous study has demonstrated that only hydrated ions of K', Rb', Cs', and NH4+ can be expected to transport through the crystal of PB during the reduction of the high-spin iron ions, and experimental results agree completely with this expectation.8c From consideration of its size, the oxygen molecule should also be able to transport through the crystal of PB. The other important aspect of the structure of PB, Fe43+[Fe11(CN)6]3.xHz0, with respect to the reduction of 02, seems to be the positions of high-spin irons, Fe3+, in the crystal (see Figure 1). The octant of the unit cell has four atoms of Fe3+ and 4 X 3/4 atoms of Fe" at positions 4a and 4b, respectively.]" The above circumstances strongly encouraged us to explore its potential application as a catalyst for the reduction of 0,. It can reasonably be expected that PB and its relatives have the capability of delivering two or four electrons, more or less simultaneously, to the 0, molecule in the crystals. It has been briefly shown in our previous paper that an autoxidation of the reduced form of PB (Prussian white) to PB occurs in an oxygen-saturated solution.& In this paper, it will be disclosed that the reduced form of PB, Prussian white, does have a catalytic activity for the reductions of O2and of hydrogen peroxide (H,02) and, also, that the oxidized form of PB shows a catalytic activity for the oxidation of H 2 0 2 . Two kinds of electron-transfer channels in the PB crystal due to the redox reactions of the high-spin iron ions, Fe3+/2+,and of the low-spin iron ions, Fe"'/I', work as a catalyst for the reduction and oxidation of Hz02,respectively. Cyclic voltammetry (CV) and a rotating ring-disk electrode (RRDE) were used for the experimental approach to be described.
Experimental Section The PB films on glassy carbon (GC-20; Tokai Carbon Co.) electrodes were prepared in an aqueous ferric ferricyanide solution of an equalvolume mixture of 20 mM FeCI3and 20 mM K3Fe(CN)6in 0.01 M HCI, as previously described.8c A large GC plate was used as a counter electrode. The electrodes were cathodically polarized in the above ferric ferricyanide solution by means of a galvanostatic condition where the current density was set between 3 and 50 FA/cm*. Mainly, a current density of 5 rA/cm2 was used. The electrodes, after the deposition of a certain amount of PB, were rinsed in 0.01 M HCI for 1 min. All the PB-modified electrodes were first examined in 1 M KCI (pH 3.0) under a nitrogen atmosphere by repeating the potential scan between 0.6 and -0.2 V where only the high-spin iron ions are involved in the electrontransfer reactions. After yielding steady voltammograms, they were subjected to further experiments as catalysts. During this potential excursion with the scan rate of 20 mV/s, the amount of PB deposits was measured by coulometry; the total amount of charge consumed by the reduction of the deposit was counted. The concentration of the hydrogen peroxide stock solution was determined by titration with standard potassium permanganate." All chemicals were of analytical
(eT)
(IO) Herren, F.; Fischer, P.; Ludi, A,; Halg, W. Inorg. Chem. 1980, 19, 956.
0 1984 American Chemical Society
Itaya, Shoji, and Uchida
3424 J . Am. Chem. SOC..Vol. 106, No. 12, 1984
rl -1 -
0-
1 -
Figure 1.
\
Illustrative depiction of the unit cell of Prussian blue
1.0
0 V i s SCE
(Fezt[Fe*1(CN)6]3.xH20). Water molecules (xH,O) and cyanide ions are omitted.
'
( 0 ,Fe3+,0,Fe".)
grade, and solutions were prepared with water that was deionized and then distilled twice in an all-glass still. Rotating GC disk electrodes used have been described previously.se For the ring-disk electrode measurements, the disk was made of GC (diameter 0.30 cm) with platinum as the ring (inner diameter 0.33 cm). The assembly was airtight and used Teflon rods as spacers. The ringdisk electrode was calibrated with the ferrocyanide/ferricyanidecouple, yielding a collection efficiency, N , of about 0.15. The PB film was prepared only on the surface of the GC disk electrode by the electrochemical method described above. During the electrochemical preparation of the PB film on the disk electrode, deposition of a small amount of PB (less than 3 mC/cm2) was not avoided on the Pt ring electrode because of the chemical deposition of PB as described previously.sc However, the PB films on the ring electrode were completely swept out by repeating the electrode potential scan of the ring electrode between the potentials where hydrogen and oxygen evolution was observed. Because the ring electrode was used for the detection of intermediate (H,O,) in the course of the reduction of 02,the small amount of PB on the ring electrode did not affect the experimental results obtained by RRDE, even if PB was present on the Pt ring. It is well-known that the current response of Pt electrodes toward H20, oxidation shows a gradual loss of potency.5a However, it will be shown in this paper that PBmodified electrodes have a high catalytic activity for hydrogen peroxide, not only for reduction but also for oxidation. An electrode rotator (Nikko-RRD-1) was used. Voltammograms were obtained with PAR (Princeton Applied Research) Model 174 instruments equipped with a Model 179 digital coulometer. A saturated calomel electrode (SCE) was used as the reference electrode. The catalytic activities of the PB-modified GC electrode were examined in the solutions of 1 M KCI and of 0.5 M K2S04whose pH values were adjusted by HCI and H2S04,respectively. Experiments were conducted at a temperature of 20 (*1) OC.
Results and Discussion Catalyzed Molecular Oxygen Reduction. Figure 2A shows the cyclic voltammogram of a PB-modified GC electrode in 1 M KC1 (pH 3.0) under a nitrogen atmosphere. The film of PB was prepared at a low current density of 5 kA/cmZ. The shape of the voltammogram is typical for the films prepared at lower current densities (
